Such a position-measuring device, designated as an interferometer, includes: a first radiation source for emitting electromagnetic beams of rays, e.g., in the form of light; a beam splitter which subdivides the beams of rays emitted by the radiation source in each case into at least a first and a second partial beam of rays, which are oriented parallel to one another after exiting from the beam splitter; a reference reflector arranged in the beam path, of the first partial beam of rays, designated as the reference beam path, by which the first partial beam of rays is reflected back; a measuring reflector that is movable with reference to the reference reflector along the measuring direction, which is arranged in the beam path of the second partial beam of rays designated as measuring beam path and reflects this back; a superposition device for superposing the two partial beams of rays after their reflection at their respectively assigned reflector for generating a measuring signal from which changes in the relative position of the measuring reflector with respect to the reference reflector are derivable; a second radiation source used to generate a reference pulse for emitting additional electromagnetic beams of rays; and a combining device by which the beams of rays generated by the second radiation source are combined into the beam path of the first radiation source, and before the beam splitter of the position-measuring device.
In order to determine the position of two objects that are movable with respect to each other using such a position-measuring device in the form of an interferometer, one of the two objects is connected to the reference reflector and the other object is connected to the measuring reflector that is movable with respect thereto. In response to a movement of the two objects, and thus also the two reflectors with respect to each other, there takes place a periodic change in the measuring signal formed by the superposition of the two partial beams of rays, in the form of an (incremental) interference signal, so that changes in the position of the two reflectors, and thus of the two objects to be measured with respect to each other may be determined with great accuracy. In such a measuring operation, the position-measuring device is operated exclusively using the electromagnetic radiation of the first radiation source, used for generating the incremental interference signal (measuring signal).
In a practical application, if perhaps an interferometer is to be used as an installation measuring system in a machine tool for determining the position of two machine parts that are movable with respect to each other, it is frequently necessary, in the determination of the position, to be able to produce an absolute reference to a specified position of the two reflectors in the measuring range of the interferometer. To do this, it is described, for example, in European Published Patent Application No. 1 068 486, that at least one reference marking may be provided at the reference reflector or at the measuring reflector, which is scannable by a scanning unit arranged at the respectively other reflector, in order to generate a reference pulse signal if the measuring reflector and the reference reflector have a specified position with respect to each other determinable by scanning the reference marking.
A further possibility for generating a reference pulse is to assign a second radiation source to the position measuring device, which emits electromagnetic beams of rays that are used exclusively for purposes of generating a reference pulse, and which for this too (within the scope of reference travel of the position-measuring device) are combined into the beam path of the position-measuring device. For this situation, it is described in Japanese Published Patent Application No. 7-190712 a position-measuring device in the form of an interferometer, in addition to a coherent radiation source (laser) used for generating a measuring signal, may be equipped with an incoherent light source, for example, a superluminescence diode, whose wavelength deviates from that of the laser light, and whose light is combined into the interferometer in addition to the laser light. After passing through the interferometer, the signal beams of rays, that are superposed on each other, of the coherent first radiation source (laser) on the one hand, and the incoherent second radiation source (superluminescence diode) on the other hand, are separated selectively as to wavelength using filters, so that, on the one hand, an interference signal for the position evaluation generated by the laser light that is used as the measuring signal, and, on the other hand, a reference signal used for generating a reference pulse, that is separate from the former and goes back to the light of the incoherent, second radiation source, are present.
Additional position-measuring devices arranged as interferometers, are described in Japanese Published Patent Application No. 5-149708 and Japanese Published Patent Application No. 11-237209, in which, for the generation of a reference pulse using a second radiation source, additional electromagnetic beams of rays are combined into the beam path of the position-measuring device and are subsequently evaluated separately, that is, independently from the actual measuring signal.
Conventional interferometers in which an additional radiation source is used for the generation of a reference pulse whose electromagnetic radiation is analyzed, separated from the actual measuring signal after passing through the interferometer, and separately, may have the disadvantage that additional components may be required, on the one hand for separating of the additional electromagnetic radiation from the measuring signal, and on the other hand for the detection of this radiation.